Among internal combustion engines mounted on vehicles, there are those which are provided with an air-fuel ratio control device. The air-fuel ratio control device includes a sensor which is disposed at an exhaust passage to detect, for example, oxygen concentration as a component value of the exhaust gas, a feedback control being effected such that the air-fuel ratio is brought into a target value by adjusting a quantity of fuel and/or a quantity of air with reference to a feedback control value calculated based on a detection signal outputted from the oxygen sensor, in order to enhance purification efficiency of the exhaust gas by a catalytic member, thereby reducing the harmful component value of the exhaust gas.
An air-fuel ratio control device for an internal combustion engine of the type mentioned above is disclosed in a Japanese Early Laid-Open Patent Publication Sho 61-234241. This air-fuel ratio control device comprises a first oxygen sensor disposed at an exhaust passage on an upstream side of a catalytic member which is disposed at the exhaust passage of an internal combustion engine, and a second oxygen sensor disposed at the exhaust passage on a downstream side of the catalytic member, a skip amount of a first feedback control value, which is calculated with reference to a first detection signal outputted from the first oxygen sensor, being corrected by a second detection signal outputted from the second oxygen sensor, in order to prevent a lowering of responsiveness due to deterioration of the first oxygen sensor. More specifically, a skipping amount calculating means calculates a skipping amount as an air-fuel ratio feedback control constant in accordance with the output of the downstream sensor, and an air-fuel ratio correcting amount calculating means calculates an air-fuel ratio correcting amount in accordance with an output of the upstream sensor using the skipping amount. An air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine in accordance with the air-fuel ratio correcting amount in order to prevent a lowering of responding speed.
In such an air-fuel ratio control device for an internal combustion engine comprising a first oxygen sensor disposed at an exhaust passage on an upstream side of a catalytic member which is disposed at the exhaust passage of an internal combustion engine, and a second oxygen sensor disposed at the exhaust passage on a downstream side of the catalytic member, the first feedback control is effected such that the air-fuel ratio is brought into a target value with reference to the first feedback control value (OXFB) calculated with reference to the first detection signal outputted from the first oxygen sensor as shown in FIG. 5, and the second feedback control is effected in order to correct reverse delay time (D.sub.LR, D.sub.RL) of a first feedback control value (OXFB) at the time when the first detection signal is reversed between a rich signal and a lean signal as shown in FIG. 7, with reference to a second feedback control value (SOXFB) which is calculated based on the second detection signal outputted from the second oxygen sensor as shown in FIG. 6, so that the feedback control to be effected with the aid of the first oxygen sensor is prevented from being shifted from the control center. The air-fuel ratio reverse delay time (D.sub.LR, D.sub.RL) when used herein refers to the time required for effecting a procedure for regarding that a change is delayed for a predetermined time when the first detection signal is changed from a rich signal to a lean signal or when the first detection signal is change from a lean signal to a rich signal.
Also, in this air-fuel ratio control device, the second detection signal outputted from the second oxygen sensor is skipped based on the respective skip values (S.sub.RL, S.sub.LR), when the second detection signal is reversed between the rich signal and the lean signal, and respective durations of time (T.sub.R, T.sub.L) of the rich signal and lean signal of the second detection signal are judged every integral value judging time t.sub.k, and an integral value (I.sub.RL) determined based on the duration of time T.sub.R /T.sub.L is increased/decreased every integral value judging time t.sub.k.
However, in an air-fuel ratio control device of the type mentioned above, as shown in FIGS. 2 to 4, a cycle or a frequency (hereinafter referred to as the "cycle") (T.sub.RE) of the second detection signal from the second oxygen sensor is changed in accordance with deterioration of the catalytic member with respect to a cycle or frequency (T.sub.FR) of the first detection signal from the first oxygen sensor. That is, when the catalytic member is deteriorated, the cycle (T.sub.RE) of the second detection signal is reduced (or becomes short) at the time when the catalytic member is deteriorated as shown in FIG. 4 relative to the cycle (T.sub.RE) of the second detection signal at the time when the catalytic member is deteriorated as shown in FIG. 3, and it is brought to be closer to the cycle (T.sub.FR) of the first detection signal of the first oxygen sensor. As a result, the respective durations of time (T.sub.R, T.sub.L) of the rich signal and lean signal are changed at the time when the catalytic member is new and therefore not deteriorated, and at the time when the catalytic member is used for a long period of time and therefore deteriorated.
If the integral value judging time t.sub.k is kept constant against change in respective durations of time (T.sub.R, T.sub.L) of the rich signal and lean signal of the second detection signal due to deterioration of the catalytic member, the second feedback control value (SOXFB) obtainable with the aid of the second oxygen sensor is extensively changed both at the time when the catalytic member is not deteriorated and when the catalytic member is deteriorated. It gives rise to the problem that the air-fuel ratio reverse delay time (D.sub.LR, D.sub.RL) of the first feedback control value (OXFB), which is obtainable with the aid of the first oxygen sensor, is shifted.
As a result, there are such disadvantages that since the first feedback control to be effected with the aid of the first oxygen sensor is extensively shifted from the control center, the first feedback control cannot be effected accurately in order to bring the air-fuel ratio into the target value with the aid of the first oxygen sensor, and purification efficiency of the exhaust gas is deteriorated, thus making it impossible to reduce the harmful component value of the exhaust gas.
If, as mentioned above, the second feedback control is effected to correct the air-fuel ratio reverse delay time (D.sub.LR, D.sub.RL) of the first feedback control value (OXFB) at the time when the first detection signal is reversed between the rich signal and the lean signal, as shown in FIG. 7 with reference to the second feedback control value (SOXFB) which is calculated based on the second detection signal outputted from the second oxygen sensor, it gives rise to another problem that the first feedback control to be effected with the aid of the first oxygen sensor is excessively sensitively responded to the second feedback control to be effected with the aid of the second oxygen sensor.
Therefore, since the first feedback control to be effected with the aid of the first oxygen sensor is excessively sensitively responded to the extensive change of the second feedback control/SOXFB), the first feedback control to be effected with the aid of the first exhaust sensor cannot effect a stable response thereto. As a result, the first feedback control to be effected with the aid of the first oxygen sensor is shafted from the control center, and the first feedback control cannot be effected accurately in order to bring the air-fuel ratio into the target value with the aid of the first oxygen sensor, with the results that the purification efficiency of the exhaust gas is deteriorated thereby making it impossible to reduce the harmful component value of the exhaust gas.
More specifically, if the integral value judging time t.sub.k is constant with respect to change in the respective durations of time (T.sub.R, T.sub.L) of the rich signal and lean signal of the second detection signal due to deterioration of the catalytic member, when the integral value judging time t.sub.k becomes longer than the duration of time t.sub.a (t.sub.k .gtoreq.t.sub.a) of, for example, the rich signal of the second exhaust detection signal due to deterioration of the catalytic member as shown in FIG. 8, the second detection signal is not reversed within the integral value judging time t.sub.k.
In this way, when the integral value judging time t.sub.k becomes longer than the duration of time t.sub.a (t.sub.k .gtoreq.t.sub.a), a judgment as to whether the second detection signal is the rich signal or the lean signal cannot be made. As a result, as shown in FIG. 9, the second oxygen feedback control value (SOXFB) obtainable with the aid of the second oxygen sensor cannot be changed to either side because of no generation of the integral value, and skipped in the neighborhood of the current value only based on the skip values (S.sub.RL, S.sub.LR). Therefore, as shown in FIG. 10, when the second feedback control is effected in order to correct the air-fuel ratio reverse delay time (D.sub.LR, D.sub.RL) of the first feedback control value (OXFB) based on the second feedback control value (SOXFB), the air-fuel ratio reverse delay time (D.sub.LR, D.sub.RL) of the first feedback control value (OXFB) obtainable with the aid of the first oxygen sensor is shifted, and the air-fuel ratio obtainable by the first feedback control is shifted from the control center where .alpha.=1. As a result, the first feedback control cannot be effected accurately in order to bring the air-fuel ratio into the target value with the aid of the first oxygen sensor, with the results that the purification efficiency of the exhaust gas is lowered, thereby making it impossible to reduce the harmful component value of the exhaust gas.
Furthermore, as shown in FIGS. 11 and 12, when the catalytic member is not deteriorated because the member is new, the integral value (I.sub.RL) is generated frequently to increase the amount of integration because the cycle of the second detection signal is long. As a result, the second feedback control value (SOXFB) is extensively changed. Since the second feedback control is effected in order to correct the air-fuel ratio reverse delay time (D.sub.LR, D.sub.RL) of the first feedback control value (OXFB) in accordance with the second feedback control value (SOXFB) which is extensively changed as mentioned, the air-fuel ratio reverse delay time (D.sub.LR, D.sub.RL) of the first feedback control value (OXFB) is uselessly changed to a long side or a short side. As a result, the first feedback control to be effected with the aid of the first oxygen sensor is excessively sensitively responded to the second feedback control, and the first feedback control cannot be responded stably. As a consequence, the air-fuel ratio obtainable as a result of the first feedback control is shifted from the control center where .alpha.=1, and the first feedback control cannot be effected accurately in order to bring the air-fuel ratio into the target value with the aid of the first oxygen sensor, with the result that the purification efficiency is lowered, thereby making it impossible to reduce the harmful component value of the exhaust gas.
Furthermore, to set the integral value judging time t.sub.k increases the capacity of a control soft and increases the cost, and is thus economically disadvantageous.
The description of the aforementioned conventional control device is described in additional detail relative to FIGS. 2 to 12 on Attachment A.
In view of the above, the present invention in a first embodiment provides, in order to obviate the above inconveniences, an air-fuel ratio control device for an internal combustion engine comprising a first exhaust sensor disposed at an exhaust passage of the internal combustion engine on an upstream side of a catalytic member disposed at said exhaust passage, a second exhaust sensor disposed at said exhaust passage on a downstream side of said catalytic member, a first feedback control being effected such that an air-fuel ratio is brought into a target value with reference to a first feedback control value which is calculated with reference to a first detection signal outputted from said first exhaust sensor, a second feedback control being effected in order to correct said first feedback control value with reference to a second feedback control value which is calculated with reference to a second detection signal outputted from said second exhaust sensor, wherein said air-fuel ratio control device for the internal combustion engine is characterized in that it further comprises control means for feedback-controlling the air-fuel ratio by changing a correction judging time and a correction amount of said second feedback control of said second exhaust sensor in accordance with an output cycle of said second detection signal from said second exhaust sensor and calculating a second feedback control learning value with reference to an arithmetical mean which is calculated with reference to a value just before the preceding skip and a value just before a current or present skip every time said second feedback control value is skipped, and an arithmetical mean number which is calculated in accordance with a cycle state of the output of said second detection signal from said second exhaust sensor.
According to this embodiment of the invention, by virtue of the control means as aforesaid, the air-fuel ratio is feedback-controlled by changing a correction judging time and a correction amount of the second feedback control of the second exhaust sensor in accordance with an output cycle of the second detection signal from the second exhaust sensor and calculating a second feedback control learning value with reference to an arithmetical mean which is calculated with reference to a value just before the preceding skip and a value just before a present skip every time the second feedback control value is skipped, and an arithmetical mean number which is calculated in accordance with a cycle state of the output of the second detection signal from the second exhaust sensor.
The present invention in a second embodiment provides, in order to obviate the above inconveniences, an air-fuel ratio control device for an internal combustion engine as described above, characterized by a control means for effecting a second feedback control such that an arithmetical mean is calculated with reference to a value just before a preceding skip and a value just before a current or present skip every time said second feedback control value is skipped, an arithmetical mean number is calculated based both on a cycle of said first detection signal and a cycle of said second detection signal in accordance with deterioration of said catalytic member, and a learning value of the second feedback control of said second exhaust sensor is calculated with reference to said arithmetical mean and arithmetical mean number, in order to correct a reverse delay time of the air-fuel ratio of said first feedback control value with reference to such calculated learning value of the second feedback control.
According to the latter construction of the invention, a second feedback control is effected by the control means such that an arithmetical mean of a value just before a preceding skip and a value just before a current skip is calculated every time the second feedback control value is skipped with the aid of the second exhaust sensor and an arithmetical mean number is calculated based both on a cycle of the first detection signal and a cycle of the second detection signal in accordance with deterioration of the catalytic member and a learning value of the second feedback control of the second exhaust sensor is calculated with reference to the arithmetical mean and arithmetical mean number, in order to correct a reverse delay time of the air-fuel ratio of the first feedback control value with reference to such calculated learning value of the second feedback control. Accordingly, the first feedback control value obtainable with the aid of the first exhaust sensor can be corrected in accordance with deterioration of the catalytic member, and the first feedback control to be effected with the aid of the first exhaust sensor can be stably responded to the second feedback control to be effected by the second exhaust sensor, without being excessively sensitively responded thereto.
The present invention in a third embodiment provides, in order to obviate the above inconveniences, an air-fuel ratio control device for an internal combustion engine as described above, characterized by a control means in which when the air-fuel ratio is feedback-controlled, an integral value judging time for the feedback-control of the rear exhaust sensor is found based on a detection value indicating an output state of the front exhaust sensor and another detection value indicating an output state of the rear exhaust sensor which is changed in accordance with deterioration of the catalyst converter, and an integral amount is found, and the air-fuel ratio is feedback-controlled in accordance with the deterioration of the catalyst converter based on such obtained integral value judging time and integral amount.
By virtue of this latter construction of the invention, when the air-fuel ratio is feedback-controlled, the feedback controlling integral value judging time of the rear exhaust sensor is found with reference to the detection value indicating the output state of the front exhaust sensor which is inputted to the control means, and another detection value indicating the output state of the rear exhaust sensor which is varied in accordance with deterioration of the catalyst converter, and the integral value is found, and the air-fuel ratio is feedback-controlled in accordance with deterioration of the catalyst converter based on the integral value judging time and integral amount, thereby enhancing purification efficiency of the exhaust gas of the catalyst converter.
Embodiments of the present invention will now be described in detail with reference to the drawings.